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Questions  

If the coefficients of (2r + 4)th and (r  2)th terms in the expansion of (1+x)18 are equal, then value of r is

a
5
b
6
c
7
d
9

detailed solution

Correct option is B

18C2r+3=18Cr−3⇒2r+3=r−3 or 18=(2r+3)+(r−3)⇒ r=−6 or r=6

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